Method of formation fracture dimensions

ABSTRACT

The invention is intended to determine the dimensions of the cracks resulting from the formation fracturing. To determine the crack dimensions a numerical model of the fracturing fluid ousting from the crack and filtrate zone with the formation fluid is made for the set formation parameters, fracturing data and supposed crack dimensions, the model is made to calculate the change of the fracturing fluid concentration in the total production during the well post-fracturing commissioning; during the well commissioning, throughout the entire fracturing fluid ousting period samples of the fluid produced are periodically taken from the well mouth and the fracturing fluid concentration in the samples taken is measure, then the measurement results are compared with the model calculations and the crack length is determined based on the best match of the measurement results and the model calculations. If polymer-based fluid is used as fracturing fluid, the numerical model also includes the change of the polymer concentration in the fracturing fluid withdrawn as function of time and in the samples taken additionally the polymer concentration is measured and by comparing the measurement results with the model calculations the crack width is determined.

This application claims the benefit of priority to International Patent Application No. PCT/RU2007/000272 filed May 29, 2007 and Russian Patent Application No. 2006118852 filed May 31, 2006, which is herein incorporated by reference.

The invention is related to the formation fracture monitoring methods and particularly is intended to determine the dimensions of the cracks resulting from the formations fracturing and may be applied in oil and gas fields.

Formation fracturing is a well-known method to intensify hydrocarbons well production by increasing producing formation bottom-hole area permeability by means of fracturing. During the formation fracturing activities the high-viscosity liquid (also known as fracturing fluid) containing proppant is pumped into the bed in order to create a crack in the production range and fill the crack with proppant. To ensure efficient use the crack must be located inside the production range and not to protrude in the adjacent strata as well as have sufficient length and width. Therefore, crack dimensions determination is a critical stage to ensure fracture process optimization.

Currently the cracks geometry is determined using various technologies and methods. Best known are the methods (so-called fracturing visualization), ensuring assessment of spatial orientation of the crack and its length during the fracturing activities and mostly based on localization of seismic phenomena using passive acoustic emissions. This localization is ensured by the “cloud” of acoustic phenomena, stating the scope within which the crack may be positioned. These acoustic emissions are microseisms resulting from either high pre-fracture stress concentration, or reduction of the current stress around the crack with the subsequent fracturing fluid flowing into the bed. At the best these phenomena are analyzed to obtain information of the source mechanism (energy, displacement field, stress drop, source dimensions etc.). Upon the results of these phenomena analysis it is impossible to obtain direct quantitative information concerning the main crack. Other methods are based on measuring soil minor deformation using dipmeters either from the surface or from the well bore. All these methods are rather expensive due to the necessity of proper positioning of the sensor in the set location accounting for the relevant mechanical grip between the bed and instruments. Other methods ensure approximate assessment of the well crack height based either on temperature variations or on the data obtained using isotopic tracers (tracer atoms). Review of the aforementioned visualization methods above is presented, e.g., in the following publication: Barree R. D., Fisher M. K.

Woodroof R. A. (2002) A practical Guide to Hydraulic Fracture Diagnostic Technologies, SPE material, paper 77442, presented at Annual Technological Conference and Exposition in San Antonio, Tex., Sep. 29-Oct. 2, 2002.

The closest analog of the method claimed is the method of bed fracture crack dimensions determination, described in the USSR Certificate of Authorship No. 1298376, 1987, and providing injection of fracture fluid in the well bore under pressure enabling the said fluid creating cracks near the well and penetrate them and further across the crack surfaces into the bed filtration zone near the crack, and subsequent fluid flow parameter measurement. This method's disadvantage is the necessity to use additional equipment and complicated calculations.

The purpose of the claimed invention is the creation of the method to determine the dimensions of the crack resulting from the bed fracturing activities based on the analysis and simulation of the fracturing fluid pumping out after the bed fracturing.

The said purpose is attained by means of creating a numerical model of the fracturing fluid pressurization from the crack and filtrate zone around the crack using formation fluid for the set formation parameters, fracturing data and supposed crack dimensions in order to modify the fracturing fluid in the total production during the well post-fracturing commissioning; during the well startup, throughout the entire period of the fracturing fluid ousting periodically fluid samples are taken from the well mouth, fracturing fluid concentration in the samples is measured and then the measurement results are compared with the numerical simulation data and the crack length is determined based on ensuring the best match of the measurement results and model calculations.

As fracturing fluid polymer fluid may be used; in this case during the numerical model creation polymer concentration change in the ousted fracturing fluid is also calculated as function of the time; in the fracturing fluid samples additionally polymer concentration is determined and, by comparing the measurement results with the model calculations, the crack width is determined.

Fracturing fluid may also contain an indicator allowing to differentiate it from the formation water in case of significant amount of the formation water present in the total production after fracturing.

In accordance with this invention determination of the crack dimensions, namely—its length and width, is based on the results of the withdrawn fracturing fluid measurement results analyzed based on the simulation of the crack cleaning of the fracturing fluid. Crack cleaning is the process of ousting (withdrawal) of the fracturing fluid from the crack and filtrate zone around the crack using the formation fluid. The analysis of the ousted fracturing fluid is the measurement of the fracturing fluid concentration in the total production as function of time after the fracturing, and, in case of using polymer fracturing fluid,—concentration of the polymer in the withdrawn fracturing fluid.

During the formation fracturing activities the fracturing fluid filtrate (or aqueous base of the fracturing fluid, in case of using polymer fracturing fluid) penetrates the formation. Simultaneously, the polymer component of the fracturing fluid (in case of polymer fracturing fluid) is held on the formation surface and stays within the crack. During the well development after the fracturing the fracturing fluid is ousted from the crack and filtrate zone near the crack with the filtration fluid. Thus, during the well commissioning after the fracturing first produced will be the fracturing fluid pumped into the formation during the fracturing activities.

Nature of the fracturing fluid concentration in the total production as function of time is directly determined by the process of the crack cleaning and filtrate area around it. Change of the ratio of the withdrawn fracturing fluid to the formation fluid in the total production depends on the rate of the fracturing fluid filtrate ousting from the filtrate zone, and, consequently, of the rate of the formation fluid penetration in the crack (across the filtrate zone) and coming out to the surface. Duration of the fracturing fluid filtrate ousting from the filtrate zone depends on the filtrate zone depth which, in its turn, depends on the crack length with the set pumped in volume of the fracturing fluid. Therefore, change of the fracturing fluid concentration in the total production with the set well yield depends on the crack length. Thus, with the equal total volume of the fracturing fluid filtrate in the filtrate zone in the early post-fracture production period the fracturing fluid concentration drops faster in the longer crack.

In case of using polymer fracturing fluid during the crack cleaning the fracturing fluid filtrate also mixes with the polymer component present inside the crack during the fracturing fluid filtrate flowing from the filtrate zone into the crack. Change of the polymer (e.g., guar) concentration inside the crack and, ultimately, in the withdrawn fracturing fluid, depends on the fracturing fluid filtrate inflow into the crack and on the polymer weight in the certain point inside the crack. On the one hand, the volume of the fracturing fluid filtrate coming from the filtration zone depends on the filtrate zone depth, and, consequently, on the crack length. On the other hand, with an equal polymer concentration along the entire crack volume the polymer weight distribution along the crack length is proportional to the crack width. Therefore, the change of the polymer concentration in the withdrawn fracturing fluid during the crack cleaning depends both on the crack length and width.

The invention is clarified by the drawings.

FIG. 1 shows the change of the ratio of the fracturing fluid withdrawal rate Q_(f) to the total well yield Q (i.e. in effect—change of water content) as function of time (time t on the Ox axis is shown in hours) for typical formation fracturing activities in Western Siberia. Solid line corresponds with the calculation for the crack with the length of 150 meters and width 5 mm, dotted line—for the crack with the length of 150 meters and width 2.5 mm, dot-and-dash line—for the crack with the length of 220 meters and width 5 mm;

FIG. 2 shows the results of the calculation of polymer concentration C in the withdrawn fracturing fluid change (in g/l) for the same dimensions as the cracks in FIG. 1 (time t on the Ox axis is shown in hours);

FIG. 3 shows the results of calculation and measurement of the change of relation of the fracturing fluid withdrawal rate Q_(f) to the total well yield Q as function of time (time t on the Ox axis is shown in hours);

FIG. 4 shows the results of calculation and measurement of the change of polymer concentration C in the withdrawn fracturing fluid (in g/l) (time t on the Ox axis is shown in hours).

The claimed method of the formation fracture crack dimensions determination is performed as follows. In the well bore fracturing fluid is pumped in, the fluid generally is a water-based high-viscosity fluid. The fracturing fluid is pumped in with the pressure sufficient to create a crack in the bottom-hole area. During the fracturing the fracturing fluid filtrate also penetrates the formation around the crack across the crack surface. The fracturing fluid may also contain an indicator allowing differentiating it from the formation water, in case of the presence of the significant amount of the formation water in the total production after the fracturing; the indicators may be represented by non-radioactive chemicals widely applied to assess water spillovers (breakthroughs) between the wells.

In case of using polymer fracturing fluid it is critical that during the pump-in only water base of the fluid flows into the formation whereas the polymer molecules due to their large size cannot penetrate the formation and stay inside the crack. Therefore, at the time of the production start of the fracturing fluid back onto the surface, the entire amount of the previously pumped-in polymer is inside the crack and the crack itself is surrounded by the fracturing fluid water base.

Samples of the fluid produced are taken during the well commissioning after performing the formation fracturing activities. Samples are taken near the well mouth using the method similar to the one usually applied to determine water content. Samples are take periodically throughout the entire period of the fracturing fluid ousting. For example, for typical post-fracturing well in Western Siberia the duration of the fracturing fluid withdrawal normally is 2-3 days, over this period product sampling is preferably made every 30 minutes during the first 7-10 hours, then—every hour throughout the remaining time. Then the samples are sent to the laboratory to measure the concentration of the withdrawn fracturing fluid in the produced fluid and the polymer concentration (for polymer fracturing fluids) in the withdrawn fracturing fluid.

In the laboratory the samples are processed in a centrifuge to separate the fracturing fluid from the oil, in the way similar to the standard water content measurement. It enables determination of the fracturing fluid content change in the total production throughout the withdrawal period reviewed. If polymer fracturing liquid was used, the fracturing fluid separated from the oil is analyzed to measure the polymer concentration. In case of using guar polymer the methodology is based on the known method applying phenol and sulfuric acid. As a result the dependence of the polymer concentration in the withdrawn fracturing fluid on the time is obtained.

To assess the crack dimensions numerical model of the fracturing fluid ousting from the crack and filtrate zone with the formation fluid is used (see, for example, Entov V. M., Turetskaya F. D., Maksimenko A. A, Skobeleva A. A. “Modeling of the Fracturing Crack Cleaning Process”, Abstracts of the Reports of the 6^(th) Scientific and Practical Conference “Urgent Problems of the State and Development of Russian Oil and Gas Industry” dedicated to the 75^(th) Anniversary of Russian State Gubkin Oil and Gas University, Jan. 26-27, 2005, Section 6 “Automation, Modeling and Utility Supply for Oil and Gas Industry Processes”, pp. 12-13).

The model calculates the change of the fracturing fluid concentration in the produced fluid, and, in case of using polymer fracturing fluid,—change of the polymer concentration in the withdrawn fracturing fluid. The model input parameters look as follows:

The formation permeability and porosity, formational pressure, production interval height, formation oil viscosity.

Well yield or bottom-hole pressure during the fracturing fluid ousting.

Total volume of the fracturing fluid, weight of the polymer and weight of the proppant pumped into the formation during the fracturing activities, the proppant permeability and porosity, fracturing fluid viscosity.

Relative phase permeability values in the formation and in the pressed proppant and the crack.

Supposed length and, in case of using polymer fracturing fluid,—supposed width of the crack

The parameters stated in 1-4 must be known from the formation properties, fracturing activities plan and data on the well productivity after holding the fracturing activities. The crack length and width are determined by comparing the results of the numerical modeling and laboratory measurement of the product samples by means of making graphs, spreadsheets or computer calculations.

The crack length and width must be selected upon the results of the best approximation of two various data sets:

Measurement of the fracturing fluid concentration in the total production obtained from numerical calculations and measured in the laboratory, Change of guar polymer concentration obtained from numerical calculations and measured in the laboratory.

In case of the results non-alignment the supposed crack dimensions are updated in such a way as to obtain the best approximation of the results of the modeling calculations and measurements using, for example, least square method or any other mathematical quantitative method of approximation degree assessment.

To illustrate the method proposed an example of comparing the results of the withdrawn fluid analysis with the model calculation of the crack cleaning after the typical formation fracturing in Western Siberia. The laboratory analysis of the fracturing fluid includes measurements of the correlation of the fracturing fluid withdrawal rate and the total yield (i.e. water content) shown in FIG. 3 with a solid line and guar concentration (in g/l) in the withdrawn fracturing fluid, shown in FIG. 4 with a solid line. The results of modeling calculations of the crack cleaning of the fracturing fluid for the scenario when the supposed crack geometry is taken from the fracturing work design obtained using typical engineering software used to calculate the crack growth during fracturing activities, shown in FIGS. 3 and 4 with a dotted line. As we can see from FIG. 3-4 (the difference between the solid and the dotted lines); the measured data and the modeling results do not match very well. To obtain a better match of the measurement results with the modeling calculations (see FIG. 3-, dot-and-dash line) the crack geometry needs to be corrected as follows: the crack length must be increased by about 40% and the width must be reduced by 30%. Such a correction is well aligned with the constancy of the proppant weight inside the crack, i.e. the crack total volume remains unchanged. The modeled forecast results may be improved by applying indicators enabling to differentiate the formation water from the fracturing fluid in case of the presence of a substantial amount of the formation water in the total production after the fracturing. 

1. Method to determine the formation fracturing crack including the process of creating a fracturing crack in the borehole zone in which a portion of the fracturing fluid across the crack surface penetrates the formation producing filtrate zone around the crack; peculiar by the fact the prior to its implementation a numerical model of the fracturing fluid withdrawal from the crack and filtrate zone with the formation fluid is made up for the set formation parameters, fracturing data and supposed crack dimensions in order to calculate the change of the fracturing fluid concentration in the total production during the well post-fracturing commissioning; during the well commissioning, throughout the entire fracturing fluid ousting, period samples of the fluid produced are periodically taken from the well mouth and the fracturing fluid concentration in the samples taken iis measure, then the measurement results are compared with the model calculations and the crack length is determined based on the best match of the measurement results and the model calculations.
 2. Method according claim 1, in which polymer-based fluid is used as fracturing fluid; the numerical model also includes change of the polymer concentration in the withdrawn fracturing fluid as function of the time, in the samples taken additionally the polymer concentration is measured and by comparing the measurement results with the model calculations the crack width is determined.
 3. Method according claims 1 and 2, in which the fracturing fluid contains an indicator allowing differentiating it from the formation water, in case of the presence of a substantial amount of formation water in the total production after the fracturing. 